Gold occurs primarily as the native metal, alloyed with silver or other metals or as tellurides. It is commonly associated with the sulfides of iron, silver, arsenic, antimony and copper. Silver occurs as finely disemminated metal in rocks of hydrothermal origin as silver chloride, sulfide or tellurides and as complex sulfides with antimony and arsenic. Historical practice with ores containing native metal involve crushing, concentration of the gold or silver by gravity separation and recovery by amalgamation with mercury. Environmental concerns have resulted in abandonment of this process in most cases. Currently there are two major processes for recovery of gold and/or silver. The most widely accepted processes today involve leaching with caustic cyanide solution coupled with recovery of the metal values by concentration with zinc dust (Merrill-Crowe) or concentration of the gold and silver cyanide complexes by absorption on charcoal followed by electrowinning (carbon absorption scheme) also referred to as Carbon in Pulp (CIP). Another process recently practiced in the Soviet Union is one in which quaternary amine ion exchange resins are employed as a replacement of charcoal in the carbon absorption scheme.
In a recent publication "Selectivity Considerations in the Amine Extraction of Gold from Alkaline Cyanide Solutions" by M. A. Mooiman and J.D. Miller in "Minerals and Metallurgical Processing", August 1984 Pages 153-157, there is described the use of primary, secondary and tertiary amines to which have been added certain Lewis base modifiers such as phosphorus oxides and phosphate esters for the extraction of gold from alkaline cyanide solutions.
The leach liquors containing the gold are achieved by leaching with cyanide solutions through either the dump or heap leaching techniques. In heap leaching, the ore is placed on specially prepared impervious pads and a leaching solution is then applied to the top of the heap and then allowed to percolate down through the heap. The solution containing the dissolved metal values eventually collects along the impervious pads and flows along it to a collection basin. From the collection basin, the solution is pumped to the recovery plant. Dump bleaching is similar to heap leaching in which old mine wastes dumps which have sufficient metal value to justify processing are leached in place. Successful dump leaching requires careful control of leach solutions to prevent ground water contamination. Heap leaching is replaced by vat leaching in areas with harsh winters, wet tropical climates or limited oxygen availability. In vat leaching the crushed ore is placed in a vat which is then flooded with leaching solution. In any of the leaching methods, a cyanide leach solution is obtained from which the metal values are recovered. While cyanide leaching is in general use today, it is relatively slow and has toxic disadvantages. Other leaching solutions are being considered in which thiourea, i.e. acido thiourea is employed, and/or thiocyanates. However, cyanide solutions appear to be the reagent of choice as a primary lixiviant for gold.
In the CIP process, coconut shell activated carbon is necessary, which is in short supply and expensive. In the Merril-Crowe process zinc dust is used to precipitate gold from the clarified cyanide solution, but the cost of this process to separate gold from leach liquors is similar to that of the CIP method.
Different amine functionalities have been considered in the past in both the liquid/liquid extraction and liquid/solid extraction of gold. For liquid/solid extraction auricyanide is too strongly bound with the quaternary amines of the resins, so that stripping is difficult and requires special treatment. In addition, no selectivity of metal cyanide complexes and leach liqors is shown. Resins with weaker basic amine functionalities cannot perform well in the pH range (10-11), the pH of the common leach liqors. For liquid/liquid extraction such as the work of Mooiman and Miller, organophosphorus modifiers, i.e. trialkylphosphates are required to increase the amine basicity in order to permit efficient extraction of the gold materials. These materials must be used in large amounts. The systems still do not reach or meet the pH criteria of leach liqors.